1. Field of Invention
This invention pertains to the art of methods and apparatuses for capturing and preconcentrating certain gaseous species for analytical detection, and more specifically to methods and apparatuses for using a compact, portable thermoelectric apparatus for preconcentrating volatile organic compounds for analysis by gas chromatography.
2. Description of the Related Art
Many volatile organic compounds ("VOCs") found in ambient air are present at very low parts per billion and parts per trillion levels. To identify and quantify these compounds, researchers must employ collection techniques that preconcentrate sufficient amounts of these materials for analytical detection.
The use of cryogenic trapping to concentrate VOCs before analysis has been established as a proven technique for VOC monitoring. This method involves collecting a sample using a trap containing an inert material (e.g., glass beads) at subambient temperatures. The temperature of the trap is below the condensation temperatures for trace VOCs but above the condensation temperature for major constituents of ambient air (e.g., nitrogen and oxygen). After collection, the trap is rapidly heated, the VOCs are desorbed, and the compounds are analyzed, typically using a gas chromatographic ("GC") system.
An alternative trapping method involves the use of adsorbent materials to collect the VOCs at ambient temperatures. Popular adsorbents include Tenax-TA, silica gel, carbon molecular sieves, and activated charcoal. Desorption again is accomplished by elevating the trap temperature prior to GC analysis. Although both single adsorbent and multiple adsorbent traps are used, single adsorbent traps are unable to completely collect and/or desorb samples having a wide range of molecular weights, and may experience interference from artifacts. Multiple adsorbent traps therefore are preferred. By using selected adsorbents in a multi-bed trap configuration, collection apparatuses can be tuned to capture a wide range of compounds with minimal artifact interferences.
Typically, liquid cryogens such as nitrogen or carbon dioxide are used to cool the sample collection trap. However, large amounts of cryogen are consumed during normal operations. Use of large amounts of cryogens is costly and involves safety and logistical burdens in the handling of these materials.
Recently, several commercial GC systems have been designed to automatically preconcentrate volatile organic compounds and analyze the enriched samples. All known units, except one, make use of a liquid cryogen to facilitate operations. The non-liquid cryogenic GC system uses a thermoelectric means to cool the collection trap. However, this apparatus is an integral part of the GC system, and cannot be transferred to a different GC.
The present invention contemplates a new and improved stand-alone cold trap assembly that is simple in design, inexpensive, effective in use, and overcomes the foregoing difficulties and others while providing acceptable sample collection and preconcentration performance.